1. Field of the Invention
The present invention relates to a semiconductor element and a method for manufacture thereof, and more particularly to an insulated-gate field-effect transistor and a method for manufacture thereof.
2. Description of the Related Art
Insulated-gate field-effect transistors, which have a high voltage resistance and can operate at a large current, are known. Insulated-gate field-effect transistors are semiconductor elements. Vertical MOSFETs (Vertical Double-Diffusion MOS: also referred to hereinbelow simply as VDMOSFET) are known as such semiconductor elements.
There is a risk of breakdown in the VDMOSFET due to an overcurrent caused by rapid changes in the drain voltage thereof. The VDMOSFET developed with the object of preventing this breakdown, that is, improving the so called dv/dt breakdown resistance, have been suggested in, for example, Japanese Patent Kokai (Laid-open Application) No. 5-055592. With such VDMOSFET, the breakdown resistance is improved by preventing the formation of a parasitic transistor in the corner portion of the base layer. Also, the degree of integration is increased by using the base layer whose plane surface has a square or substantially square shape. The chip size can be reduced if the degree of integration increases.
Japanese Patent Kokai (Laid-open Application) No. 5-055584 discloses another method of fabricating the VDMOSFET. This VDMOSFET fabrication method is carried out without the mask alignment process. The mask alignment process is executed when forming a contact hole. This VDMOSFET fabrication method also eliminates the defects of step coverage caused by the steps in the contact holes. In this VDMOSFET fabrication method, the entire surface of the semiconductor substrate is covered with a phosphorus-doped oxide film, reflow is conducted, the phosphorus-doped oxide film and gate insulating film are etched over the entire surface till the silicon semiconductor substrate on the source diffusion region is exposed, and the contact hole is self-alignment formed.